1. Field of the Invention
The present invention relates to semiconductor integrated circuit devices, and in particular to output circuits that are required to transfer data at high speed over a long distance.
2. Background Art
In recent years, there has been growing demand for accurate, high-speed data transfer with regard to data transfer between semiconductor devices over transmission lines. However, when data is transferred at a speed that is greater than or equal to the order of GHz, the waveform of the data will be attenuated after such data has passed through a transmission line.
FIG. 1 illustrates an example of the loss in a transmission line. The higher the frequency is, the greater the attenuation is. For example, in FIG. 1, a wave with a frequency of 1 GHz is attenuated by −11.2 dB. This means that when a sine wave with a voltage level of 1 V is transmitted, the wave will be attenuated to 0.274 V after it has passed through a transmission line. Meanwhile, a wave with a frequency of 2.5 GHz is attenuated by −21.4 dB. This means that when a sine wave with a voltage level of 1 V is transmitted, the wave will be attenuated to 0.085 V after it has passed through a transmission line.
FIGS. 2A to 2E illustrate examples of waveforms of data before and after having passed through transmission lines. In FIG. 2A, waves output from an output driver 201 pass through transmission lines 202, and then are received by a receiver 203. Outputs PAD204 monitor output waveforms, while inputs PAD205 monitor input waveforms.
FIG. 2B illustrates an example of an output waveform at the output PAD204. This is an example of a data string of 010000 in a 1-bit isolated pattern. A rectangular wave such as the one shown in FIG. 2B includes high-frequency components at the rising and falling edges.
FIG. 2C illustrates an example of a waveform at the input PAD205. The high-frequency components in the rising and falling edges of the output waveform shown in FIG. 2B are significantly attenuated after having passed through the transmission line and thus the waveform observed at the input PAD is not rectangular any more and is distorted because the amount of wave attenuation is greater as the frequency of the wave is higher as shown in FIG. 1. As a result, data ‘1’ of a temporal region 206 in FIG. 2C adversely affects data ‘0’ of adjacent temporal regions 207 and 208 as well as subsequent temporal regions 209, 210, and 211, thus disturbing the waveform. Such a phenomenon is called inter symbol interference (ISI). In high-speed data transfer, time in which a waveform is distorted accounts for a great part of the time required for transmission of 1 bit, thus becoming an obstacle to accurate data transfer. As a means for solving such problem with ISI, an output pre-emphasis technique has been known in which a ‘0’ level and a ‘1’ level of an amplitude of an output waveform are adjusted in advance so that data that has passed through a transmission line has a waveform that is as close as possible to a rectangular wave.
FIG. 2D illustrates an example of a pre-emphasized output waveform. In FIG. 2D, distortion of the waveform in FIG. 2C is taken into consideration in advance so that an amplitude corresponding to the data of ‘0’ or ‘1’ in FIG. 2B is output.
FIG. 2E illustrates the pre-emphasized output waveform of FIG. 2D after having passed through the transmission line. As the output pre-emphasis technique has been used, a waveform with small ISI is obtained.
Such an output pre-emphasis technique has been publicly known. For example, Reference 1 (JP Patent Publication (Kokai) No. 2006-352374 A) discloses an algorithm that determines the amplitude corresponding to the output data, namely, the amount of output pre-emphasis.